Water on Mars ... again

The latest results from two robotic spacecraft show that Mars was a wet world long, long ago - and might still be today. This is continuing our decades-long question about whether Mars could be the closest place where life could exist beyond Earth.

A summary of the first year of data, collected by the Curiosity Rover, shows that the large crater it landed in was once a lake, 3.7 billion years ago.

That means the atmosphere of Mars had to be thicker and warmer in the past than it is today, when liquid water flowed on the surface for hundreds of thousands of years, possibly millions.

In other words, during the early days of the solar system, there were two blue planets - Earth and Mars - but only one of them remained that way.

Mars has been through extreme climate change, where that warm wet world entered an extreme ice age and never recovered. The atmosphere leaked off into space and the water either became permafrost or accumulated in the polar ice caps.

Today, it is a cold, dry place where a glass of water placed on the ground beside Curiosity would instantly boil away into vapour - because of the extremely low atmospheric pressure, which is only about 1 per cent of sea level on Earth. The same thing would happen to your blood if you took a walk on Mars without wearing a spacesuit.

There's a lesson there on how much the environment of a planet can change over time. But even though you can no longer go swimming on Mars today, there is still hope that liquid water may exist for at least brief periods.

Images from the Mars Reconnaissance Orbiter, looking down from high above, show dark streaks that appear on hillsides when they are bathed in sunlight during the Martian summer. Is it water oozing out of frozen layers of ice?

In any case, water - whether in the past or present - means there is a possibility that life could have existed on the Red Planet and may even persist there today. We just haven't found it ... yet.

This is not the first time this question of life on Mars has been raised. In fact, it was the belief in water on the surface of Mars that led to our fascination with that planet in the first place.

Long before robotic spacecraft began visiting our planetary neighbours, early telescopic images showed a crimson world with light and dark markings, polar caps that grow and shrink with the seasons, and - according to some astronomers - long thin lines criss-crossing the surface that could be either natural channels or artificial canals.

Canals, of course, imply engineers to build them. And if there were Martian engineers they must have been incredible, those early astronomers thought, because the canals on that planet straddled entire hemispheres, dwarfing anything on Earth. Perhaps they were needed to ship water from the Polar Regions to the arid cities near the equator.

Well, if the Martians can build enormous canals, maybe they could build spaceships. Maybe they see our beautiful blue Earth in their telescopes and have decided to come here to take over our much nicer planet ... the genesis idea for the War of the Worlds.

Of course, once we began invading Mars with our spaceships in the 1960s, we didn't find any canals, cities, or Martian civilizations. Too bad, really. That would have been much more interesting.

But there were signs of dried-up river beds running all over the planet, so Mars, indeed, does have channels. Those ancient rivers renewed the idea that Mars was once wet like the Earth, billions of years ago, around the same time that life was getting a foothold on Earth.

If life started here at that time, why couldn't it have started there as well?

The problem is that after 21 successful missions to Mars, no spacecraft has found direct evidence for life. No Martians have walked by our cameras, no footprints appear in the sand, no microbes have been spotted in the soil - not even a fossil of life from the past. Nada, zero, nothing.

Even though scientists can now show definitely that the planet was hospitable for life in the past, until we find something - even a piece of alien DNA - we will not know for sure if that planet is, or ever was, alive.

If Mars does turns out to be a barren world (there are still many places we haven't looked, such as underground), that doesn't mean life won't appear there in the future. More than 200,000 people have signed up for a one way trip on he Mars One Project. If that goes ahead as planned, we will be doing to the Red Planet exactly what we feared they would do to us: land in spaceships and take over the world.

(If you want to hear more about the latest findings from Curiosity, tune into Quirks & Quarks this weekend or download the show from our web page.)

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